1. Field of the Invention
The present invention relates to a wiring board having one or more wiring layers, and a method of manufacturing the same. A projecting bump for flip-chip connection is formed as a first terminal area on one side of the wiring layers. The bump is flip-chip connected to a semiconductor element. A second terminal area is formed by plating on the other side of the wiring layers, such that an opening of a solder resist covering the wiring layers is filled with the second terminal area.
2. Background Art
Recently, a semiconductor element (also referred to as “semiconductor chip” or simply as “chip”) of a semiconductor device has a larger number of external terminals. A semiconductor element is operated at a high speed. Thus, in order to improve the electric properties of the semiconductor element, the semiconductor element is bonded to a board for a semiconductor package as an interposer, by means of flip-chip bonding of an area array type.
In C4-type flip-chip bonding (a type of flip-chip connection by solder bonding), it has been conventionally necessary to form solder bumps on terminals of a board, for a stable bonding to solder bumps on a semiconductor element.
Such solder bumps are generally formed on a board in the following manner. First, a solder paste is screen-printed on the board using a metal mask to supply a solder thereon, and then the solder on the board is subjected to a reflow process, a flux removal process, and a planarizing process, so that solder bumps are formed.
However, screen printing of a solder paste on a board entails the high cost of manufacturing a metal mask. In addition, pitch reduction between bumps is limited. A minimum pitch is generally considered to be about 150 μm.
In the future, a semiconductor element will have a finer wiring of about 90 nm, and thus a smaller terminal pitch may be needed. Therefore, another method alternative to screen printing is sought to form bumps at smaller pitches.
A method of forming bumps at smaller pitches by a chemical reaction is one example of a method alternative to screen printing. That is, bumps are formed on a board by a substitution reaction of metal and a metal salt. However, this method also suffers from problems such as high material and manufacturing costs, and uneven heights of the bumps.
When solder bumps formed on a board by screen printing are bonded to solder bumps on a semiconductor element, the bumps are bonded by heating and melting a solder after the bumps are brought into contact with each other. The uneven heights of the solder bumps on the board may cause insufficient bonding of the solder bumps on the board to the solder bumps on the semiconductor element.
In flip-chip bonding, where the solder bumps on the semiconductor element are not melted, but the solder bumps on the board are melted, there is a problem in that oxidized surfaces of the solder bumps on the board result in a deterioration in a wetting property thereof to the solder bumps on the semiconductor element.
A board has generally a solder resist disposed thereon, and terminals to be bonded to solder bumps on a semiconductor element. When a board is of an NSMD (Non-Solder Mask Defined) structure, the terminals on the board may be displaced from the solder resist, depending on shapes of the terminals. Further, such a board can be easily damaged by a stress caused by heat or an impact caused by dropping the board. As a result, the board may be defectively bonded to a semiconductor element because of deformations of the terminals on the board, which deteriorates a reliability of the board.
In particular, smaller bump pitches leads to a smaller diameter of an opening of a solder resist. Thus, when a board is of an SMD (Solder Mask Defined) structure, a solder must be supplied to an opening of a solder resist in order to realize a satisfactory bonding of the board to a semiconductor element.
As shown in FIG. 7(a), the term “NSMD structure” herein means that a region of a terminal area 721 is not limited by a solder resist 722, while the term “SMD structure” means that a region of the terminal area 721 is limited by the solder resist 722, which is shown in FIGS. 7(b) and 7(c).
Even in the SMD structure where a solder is supplied to an opening of a solder resist, when a wall angle of the solder resist in cross-section is substantially perpendicular, stress is concentrated on an upper surface of the solder (a part indicated by D1). Thus, a duration of life of the board may be reduced, if a thermal impact is applied thereto.
On the other hand, when a wall angle of a solder resist in cross-section is obtuse, there are problems in that sizes of bottom parts of openings are not uniform depending on a thickness of a resist and a sensitivity of the resist, while sizes of surfaces of the openings cannot be enlarged when bumps are formed at smaller pitches.
Conventional methods are disclosed in Japanese Patent Laid-Open Publication Nos. 2001-93929 and 2002-203868.
As described above, in the C4-type flip-chip bonding (a type of flip-chip connection by solder bonding), solder bumps are formed on a board. Although smaller bump pitches are desired, solder bumps at such smaller pitches cannot be formed on a board by means of the conventional screen printing of a solder paste. When bumps are formed on a board by means of a substitution reaction of metal and a metal salt, there are disadvantages of high material and manufacturing costs, and uneven heights of the formed bumps. A board generally has a solder resist disposed thereon and terminals to be bonded to solder bumps on a semiconductor element. However, such a structure causes various problems, and thus measures to deal with these problems are needed.